Not very. 3.5mm is designed to carry a signal, not necessarily make it powerful. To make it loud/penatrating, you need to amplify it and that means injection 100s of watts from an outlet. For a public gatherings, you'll need a total RMS of well into the 1000s, if not 10s of thousands of watts. People en masse make, and absorb, a lot of noise and, put simply, it must be overpowered.